The present invention relates to a diagnostic system for an idle speed control valve arranged to an engine.
An automotive engine is provided with an idle speed control valve for adjusting an amount of auxiliary intake air which flows in bypassing an intake throttle valve, thus automatically setting the idle speed to a predetermined value.
If the idle speed control valve has a failure, etc., the idle speed is reduced to cause unstable operability of the engine, or inversely the idle speed is increased to cause deteriorated fuel consumption, etc. Thus, the abnormality diagnosis of the idle speed control valve is needed.
One of conventional diagnostic systems for an idle speed control valve is disclosed, for example, in JP-U 63-177648, which is constructed to detect a variation in the engine speed with a variation in the opening degree of the idle speed control valve, and determine abnormality of the idle speed control valve when a variation in the engine speed is smaller than a predetermined value.
As to such conventional diagnostic system for an idle speed control valve, however, since the engine speed can vary due to a factor other than malfunction of the idle speed control valve, it is difficult to correctly determine abnormality of the idle speed control valve.
It is, therefore, an object of the present invention to provide a diagnostic system for an idle speed control valve which allows correct diagnosis of abnormality of the idle speed control valve.